1. A molecular model of renin has been constructed based on its homology wilth the other aspartyl proteinases. 2. Crystals of tryptohpan synthetase have been examined and found to be very suitable for an X-ray diffraction investigation. 3. Neutron diffraction analyses have been carried out on bovine pancreatic trypsin inhibitor and on insulin. These studies reveal the exchangeable amide protons in the structure.